This study of hormonal regulation is targeted to define the biochemical mechanisms whereby age-dependent changes perturb physiological control systems and, thus, lead to a failure to maintain homeostasis in the aged. Investigations are focused on the biochemical interactions of hormones, which are mediated via cyclic AMP and cyclic GMP. Topics investigated include: 1) hormone receptors in membranes; 2) adenylate cyclase and guanylate cyclase activities; 3) phosphorylation of membrane proteins by cyclic nucleotide-dependent and -independent protein kinases; 4) control of cyclic nucleotide levels by regulation of phosphodiesterase activities. Hormonal regulatory systems in kidney, heart, brain, colon, aorta, and ductus deferens were studied.